This invention relates generally to optical systems that include control of the polarization of light from an independent light source, and particularly to optical treatment or measurement of a target, where the focus or image of the treatment light must be highly resolved, and where the treatment or measurement light traverses a birefringent material before reaching the target. The optical treatment may be an imaging process, or a process of working the target by means of a laser. The optical measurement may be done with coherent or incoherent light.
Although many optical treatments and measurements are performed on exposed targets, some are performed on targets buried under one or more intervening layers that are substantially transparent to the treatment beam. In this document, a “target” is any location where optical treatment or measurement is desired, whether it lies on an interface surface or within a bulk material. Bottom-surface ablation, as shown in FIG. 1, involves ablating target film 101 by sending treatment beam 102 through superstrate 103 (and sometimes other intervening films). Bottom-surface ablation has the advantage that gravity draws ablated material 104 from ablation cut 105 downward, away from the workpiece, so it does not re-deposit on the workpiece and need to be cleaned off. In addition, if the treatment or measurement beam is Gaussian, the depth of focus (DOF) in the intervening layer is n times the DOF in air, where n is the refractive index of the intervening layer. This makes the focusing or imaging of the beam on the target less sensitive to surface contours and thickness variation of the workpiece.
Other optical treatments that may be performed through intervening layers include, but are not limited to, marking (as described in Herrmann's PCT Application No. WO0061634), annealing and other structural and optochemical changes, pinhole remelting, intentional color-center formation, selective curing, and exposure of wavelength-specific resists, dyes, and other photosensitive material. Measurements may include profilometry, reflectometry, absorption, microscopy, and refractive-index measurements. Reasons for sending the treatment beam through an intervening layer may be that the target is not completely solid and needs to be contained; that the target material needs to be sealed away from ambient atmosphere; or that the treatment must be tamper-proof Measurements are often done through an intervening layer to determine whether the application of the intervening layer altered the characteristics of the underlying layer.
Spatial resolution is critical to some of these optical treatments and measurements. These treatments and measurements include forming a spatially precise pattern of light at the target; for instance, a focused spot, an image, or an interference pattern. However, if the intervening layer is birefringent, the treatment or measurement beam may be split into two polarization components propagating at different speeds and angles. These separate components form separate patterns at the target, so that the resolution of the resulting treatment or measurement is degraded.
FIG. 2 is a simplified illustration of this effect in the general case. Incident beam 202, having arbitrary polarization, enters layer 203 at incident locus 204 (shown here as a point for simplicity; for different beam 202 characteristics, incident locus 204 could be a line or an area). If layer 203 were isotropic, the entire beam would refract along ordinary path 212 and form a pattern 214 on target 201. However, if layer 203 is birefringent, its characteristics include an optic axis 210, shown here with arbitrary orientation. When beam 202 enters layer 203 at incident locus 204, it splits into two orthogonally polarized component beams: ordinary component 212 polarized in direction 213 and extraordinary component 222 polarized in direction 223. The component beams are refracted at different angles because the effective refractive index of layer 203 is different for the ordinary and extraordinary polarizations. Thus the beam forms two spatially separated patterns at the target: pattern 214 from ordinary component 212 and pattern 224 from extraordinary component 222.
Depending on the thickness, the ordinary refractive index, and the birefringent index difference of layer 203 and the incident angle, convergence angle, wavelength, and polarization characteristics of incident treatment beam 202, the superposed patterns 214 and 224 may appear on the target as a single blurred pattern or a doubled pattern, similar to an image viewed through Iceland spar crystal. Even at normal incidence, where the impact of isotropic refractive-index variations on optical treatment and measurement of buried layers is largely mitigated, a birefringent layer can still split the beams along different paths, forming a double pattern at the target, unless the optic axis happens to be parallel or perpendicular to the layer surface. For example, a beam intended to form a 25-micron spot on a target through 3 mm of glass with a birefringent index difference of 0.005 (which is fairly small) was observed to form two overlapping spots with centers separated by 10 microns at the target. Where spot resolution is important to treatment or measurement quality, this is a significant loss of resolution.
A wide range of materials in current industrial use may form a birefringent layer. Many crystalline materials, including liquid crystals, are inherently birefringent. Microcrystalline thin films may also exhibit some localized birefringence. Glasses and polymers, although usually inherently isotropic, can become birefringent from fabrication stresses (especially if fabricated in large-sheet form) or post-fabrication treatments. A prime example is tempered glass.
Tempered glass is preferred for use in glass devices that need to be durable, such as solar panels, outdoor displays, and architectural or vehicle glass. Compared to annealed glass of the same composition, tempered glass is stronger, more thermally resistant, and less hazardous in case of breakage because it breaks into small cuboid fragments rather than irregular shards of varying size. However, the residual strain from the rapid, non-uniform cooling that tempers the glass makes it birefringent. Moreover, the birefringent characteristics are not constant but vary with location on each individual sheet. Transparent polymers are also used for some of the same applications but “optical-grade” polymers that are specially constrained for low birefringence are relatively costly, especially large pieces. With polymers, too, the birefringent characteristics vary within a sheet as well as between individual sheets and from batch to batch.
“Smart” thin-film structures fabricated on glass and transparent polymer are increasingly popular in a wide variety of applications including solar panels, displays and active climate-managing windows and lighting fixtures for buildings and vehicles. Many of these products would also benefit from the safety and durability of tempered glass or the low cost and convenience of non-optical-grade polymer, if precision optical treatment and measurement were possible despite the birefringence. Therefore, a need exists for spatially precise measurement and treatment of a target through an intervening birefringent layer without the pattern-doubling effects that birefringence tends to induce.